Mohamed A. Haleem

Dynamic Spectrum Access with QoS and Interference Temperature Constraints

Spectrum is one of the most precious radio resources. With the increasing demand for wireless communication, efficiently using the spectrum resource has become an essential issue. With the Federal Communications Commissions (FCC) spectrum policy reform, secondary spectrum sharing has gained increasing interest. One of the policy reforms introduces the concept of an interference temperature - the total allowable interference in a spectral band. This means that secondary users can use different transmit powers as long as the sum of these power is less than the interference threshold. In this paper, we study two problems in secondary spectrum access with minimum signal to interference noise ratio (quality of service (QoS)) guarantee under an interference temperature constraint. First, when all the secondary links can be supported, a nonlinear optimization problem with the objective to maximize the total transmitting rate of the secondary users is formulated. The nonlinear optimization is solved efficiently using geometric programming techniques. The second problem we address is, when not all the secondary links can be supported with their QoS requirement, it is desirable to have the spectrum access opportunity proportional to the user priority if they belong to different priority classes. In this context, we formulate an operator problem which takes the priority issues into consideration. To solve this problem, first, we propose a centralized reduced complexity search algorithm to find the optimal solution. Then, in order to solve this problem distributively, we define a secondary spectrum sharing potential game. The Nash equilibria of this potential game are investigated. The efficiency of the Nash equilibria solutions are characterized. It is shown that distributed sequential play and an algorithm based on stochastic learning attain the equilibrium solutions. Finally, the performances are examined through simulations

Multiple ARQ processes for MIMO systems

We propose a new automatic repeat request (ARQ) scheme for MIMO systems with multiple transmit and receive antennas. The substreams emitted from various transmit antennas encounter distinct propagation channels and thus have different error statistics. When per-antenna encoders are used, separating ARQ processes among the substreams results in a throughput improvement. Moreover, it facilitates the interference cancellation in certain MIMO techniques. Quantitative results from UMTS simulations demonstrate that the proposed multiple ARQ structure yields more than 30% gain in link throughput.

Opportunistic Encryption: A Trade-Off between Security and Throughput in Wireless Networks

Wireless network security based on encryption is widely prevalent at this time. However, encryption techniques do not take into account wireless network characteristics such as random bit errors due to noise and burst errors due to fading. We note that the avalanche effect that makes a block cipher secure also causes them to be sensitive to bit errors. This results in a fundamental trade-off between security and throughput in encryption based wireless security.1 Further, if there is an adversary with a certain attack strength present in the wireless network, we see an additional twist to the security-throughput trade-off issue. In this paper, we propose a framework called opportunistic encryption that uses channel opportunities (acceptable signal to noise ratio) to maximize the throughput subject to desired security constraints. To illustrate this framework and compare it with some current approaches, this paper presents the following: 1) mathematical models to capture the security-throughput trade-off, 2) adversary models and their effects, 3) joint optimization of encryption and modulation (single and multirate), 4) the use of forward error correcting (FEC) codes to protect encrypted packets from bit errors, and 5) simulation results for Rijndael cipher. We observe that opportunistic encryption produces significant improvement in the performance compared to traditional approaches.

Priority Based Dynamic Spectrum Access with QoS and Interference Temperature Constraints

Spectrum is one of the most precious radio resources. With the increasing demand for wireless communication, efficiently using the spectrum resource has become an essential issue. With the Federal Communications Commissions (FCC) spectrum policy reform, secondary spectrum sharing has gained increasing interest. One of the policy reforms introduces the concept of an interference temperature - the total allowable interference in a spectral band. This means that secondary users can use different transmit powers as long as the sum of these power is less than the interference threshold. In this paper, we study two problems in secondary spectrum access with minimum signal to interference noise ratio (quality of service (QoS)) guarantee under an interference temperature constraint. First, when all the secondary links can be supported, a nonlinear optimization problem with the objective to maximize the total transmitting rate of the secondary users is formulated. The nonlinear optimization is solved efficiently using geometric programming techniques. The second problem we address is, when not all the secondary links can be supported with their QoS requirement, it is desirable to have the spectrum access opportunity proportional to the user priority if they belong to different priority classes. In this context, we formulate an operator problem which takes the priority issues into consideration. To solve this problem, first, we propose a centralized reduced complexity search algorithm to find the optimal solution. Then, in order to solve this problem distributively, we define a secondary spectrum sharing potential game. The Nash equilibria of this potential game are investigated. The efficiency of the Nash equilibria solutions are characterized. It is shown that distributed sequential play and an algorithm based on stochastic learning attain the equilibrium solutions. Finally, the performances are examined through simulations

Robust encryption for secure image transmission over wireless channels

The security of multimedia data transmitted over wireless networks is of increased interest. Encryption mechanisms securely transmit multimedia data over insecure networks. A major issue that has received very little attention so far is that the very same properties that gives ciphers (encryption mechanisms) their cryptographic strength make them sensitive to channel errors as well. In addition, this would enhance the error propagation inherent in compressed data. Therefore provision of security for multimedia transmission over wireless channel results in throughput loss. Nevertheless this lost throughput is traded for increased security. To our knowledge there has been no substantial effort to optimize this tradeoff. Opportunistic encryption proposed in this work is a way to optimize the tradeoff between security offered and the throughput lost due to a cipher. We show that opportunistic encryption methods that adapt to channel variations will lead to an overall increase in the system performance. Two broad scenarios based on channel knowledge are considered, (a) exact channel state information up to a finite time horizon is known and (b) only the average signal-to-noise ratio (SNR) is known. Proposed opportunistic encryption framework is found to achieve significant gains in throughput compared to fixed block length encryption methods for a wide range of average SNR values. We have shown that applying opportunistic encryption on JPEG compressed image results in a better quality of received image and improved security compared to fixed block length encryption.

On the distribution of ambiguity levels in MIMO radar

The statistical behavior of sidelobe-ambiguity arising in location estimation in distributed coherent MIMO radar is studied in this paper. A model is developed for analyzing the statistics of the localization metric under random sensor locations. Closed form expressions are obtained for the mean and variance of the localization metric. It is shown that the mean is independent of the number of sensors and its sidelobes decrease with the squared distance from the coherence point of the MIMO array. With M transmit sensors and N receive sensors, the variance behaves as 1/MN for locations beyond the vicinity of the target being observed. When all N sensors function as transceivers, the variance approaches 2/N2. Except in the vicinity of the target, the side lobe levels have equal distribution at every location. The study is extended to derive the statistics of the peak sidelobe level and a simple expression is obtained relating the required number of sensors for a tolerated level of peak sidelobes and a desired confidence level.

Wireless Sensor Networks for Pilgrims Tracking

Every year, and for five days, about three million pilgrims gather in the small city of Makkah, Saudi Arabia, to perform the rituals of Hajj (Pilgrimage). Tracking the movement of such a large number of people is crucial to the pilgrims themselves and the authorities managing the whole event. This letter reports a real-time pilgrim tracking system that has been designed and implemented. The system relies on a dedicated delay-tolerant wireless sensor network (WSN). This WSN is interfaced to the Internet through gateway(s) available from an internet service provider (ISP). Energy efficiency, robustness, and reliability are key factors in the design of the system. Each pilgrim is given a mobile sensor unit which includes a GPS chip, a microcontroller, and antennas. A network of fixed units is installed in the Holy area for receiving and forwarding data. Periodically, each mobile unit sends its user identification (UID), latitude, longitude, and a time stamp. A central server maps the latitude and longitude information on a geographical information system (GIS). The developed system can be used to track a specific or a group of pilgrims. The developed system was tested during the last two pilgrim seasons. The pilot system was able to successfully track all pilgrims who participated in the experiment.

Pilgrims Tracking Using Wireless Sensor Network

This paper describes a prototype of a wireless sensor network developed for tracking pilgrims in the Holy areas during Hajj (Pilgrimage). A delay tolerant network principle is used. Energy efficiency, robustness, and reliability are the key factors for the developed system. Every pilgrim will be given a mobile sensor unit that includes a GPS unit, a Microcontroller, antennas and a battery. A network of fixed master units is installed in the Holy area. Upon request or periodically, the sensor unit sends its UID number, latitude, longitude, and time. The close by master unit receives this information and passes it to a server that maps the latitude and longitude information on a Google map or any geographical information system. The developed system can be used to track specific pilgrims. Alternatively any pilgrim can request emergency help using the same system. The location of the person needing help will be identified on the map to make it easy for the help to reach in the most efficient way. The developed system works in coordination with an RFID identification system that was proposed earlier. The developed system was tested successfully during the recent pilgrim season.

Joint Distributed Compression and Encryption of Correlated Data in Sensor Networks

In this paper, we propose, formulate, and study a joint distributed data compression and encryption scheme suitable for wireless sensor networks where we adopt the structured encryption system of advanced encryption standard (AES). The distributed compression is achieved as per the Slepian-Wolf coding theorem, using channel codes. Core to achieving optimal compression in the joint compression and encryption is the preservation of correlation among different blocks of data despite applying cryptographic primitives. We establish that the correlation between sources remains unchanged when cryptographic primitives, namely key addition and substitution are applied. However, as a requirement of security in the encryption, any correlation between two inputs to a encryption system is removed with diffusion techniques. Compliance to the requirements of diffusion layer of AES cipher is achieved by designing the compression function so as to maintain branch number property. We establish the necessary and sufficient condition for achieving a compression function with branch number property and show that distributed compression using non-systematic Reed Solomon (RS) code can satisfy this condition

Multiple Antenna Enhancements for a High Rate CDMA Packet Data System

A High Data Rate (HDR) system has been proposed for providing downlink wireless packet service by using a channel-aware scheduling algorithm to transmit to users in a time-division multiplexed manner. In this paper, we propose using multiple antennas at the transmitter and/or at the receiver to improve performance of an HDR system. We consider the design tradeoffs between scheduling and multi-antenna transmission/detection strategies and investigate the average Shannon capacity throughput as a function of the number of antennas assuming ideal channel estimates and rate feedback. The highest capacities are achieved using multiple antennas at both the transmitter and receiver. For such systems, the best performance is achieved using a multi-input multi-output capacity-achieving transmission scheme such as BLAST (Bell Labs Layered Space-Time) in which the transmitted signal is coded in space and time, and the receive antennas are used to resolve the spatial interference. In the second part of the paper, we discuss practical transmitter and receiver architectures using BLAST for approaching the theoretical gains promised by the capacity analysis. Because the terminal receivers will be portable devices with limited computational and battery power, we perform a computational complexity analysis of the receiver and make high-level assessments on its feasibility. We conclude that the overall computational requirements are within the reach of current hardware technology.